Good question. It is probably my Zeus. Zeus is a very finicky iem. The worst there is. It absolutely requires a cable upgrade and the right one too and the right source with the right tips. If you get all that correct it is most definitely flagship level sonically. It took me almost a year to find just the right combo. Zeus, Azla tips,ISN CU4 cable using my M15.
Reason why there are mixed views on the EE Zeus is because I am 100% certain folks have not used the right combo and source for it. I even had it for sale on the sales threads.A true love hate relationship. But recent ISN CU4 cable changed all that. I tried literally just about all my cables using a 2 pin adapter to try to mesh well with it. Silver don't mesh well with the Zeus and not all copper cables work well with them either. It requires a higher end crystal copper and thicker variety too like the ISN CU4.
I liked the ISN CU4 so much on my Zeus, I ended up buying the 2 pin version or 2nd CU4 cable just so I can use that on it.
A close 2nd is my IBasso IT04. One of the best reference tuned IEMS at the price and a bit underappreciated on the threads. It has one of the widest stages for in ears and these are also finicky with cables and tips. Not as much as the Zeus. IT04 will give you a taste of TOTL sound without spending an arm. If you guys can see a used set on the forums I would go for it. I know there aren't much chat about them now a days but they are much better than you would imagine. These work extremely well with dynamic warmer sources like the Shanling M5s or better yet my M15.
Lastly is the JVC FXD1. I know you might read some conflicting views about this set but. These are about as good as your gonna get in the price range that is more closer to a neutral signature. These are fairly easy to drive and are not as finicky when it comes to cables and tips. But with the right cable these become very enjoyable. The FDX1 is clearly a transparent iem so cables do have an affect on the sonics. They sound fantastic amped too.